1. Field of the Invention
The present invention relates to a light amount adjustment device mounted to, e.g., a lens barrel of an image pickup apparatus such as a digital camera.
2. Description of the Related Art
In a light amount adjustment device mounted to a lens barrel of an image pickup apparatus such as a digital camera, there are sometimes provided shutter blades for opening/closing an exposure aperture and a diaphragm blade for narrowing the exposure aperture. If electromagnetic drive sources are provided independently for the shutter blades and for the diaphragm blade, the resultant device becomes high in cost and large in size, weight, and power consumption, which poses a problem.
A light amount adjustment device shown in FIG. 14 has therefore been proposed, in which shutter blades and a diaphragm blade are driven by a common electromagnetic drive source (see, Japanese Laid-open Patent Publication No. 2001-117135 and corresponding U.S. Patent Publication No. 6,903,777).
The proposed light amount adjustment device (digital camera shutter) includes a motor (electromagnetic drive source) that has a coil and a rotor 104 having a two-pole magnet. The rotor 104 rotates in a direction corresponding to the direction of coil energization. With clockwise rotation of the rotor 104, shutter blades 118, 119 are rotated by a driving pin 104b of the rotor in directions to open an aperture 101a formed in a shutter base plate 101. With anticlockwise rotation of the rotor 104, the shutter blades 118, 119 are rotated in directions to close the aperture 101a. 
The light amount adjustment device also includes a diaphragm blade 120 formed with an aperture 120a, and a diaphragm actuating member 115 coupled to the diaphragm blade 120. The diaphragm actuating member 115 is coupled to the driving pin 104b of the rotor 104 via a spring 116 and to the shutter base plate 101 via a spring 117, and is urged by the spring 117 in such a manner that an engaging portion 115a of the diaphragm actuating member 115 is brought in contact with the driving pin 104b. 
When the rotor 104 rotates clockwise, the diaphragm actuating member 115 is rotated anticlockwise by the urging forces of the springs 116, 117, so that the aperture 102a of the diaphragm blade 120 is disposed to face the aperture 101a of the shutter base plate 101. Subsequently, when the diaphragm actuating member 115 is rotated clockwise by the driving pin 104b of the rotor 104, the diaphragm blade 120 returns to the state shown in FIG. 14.
In a state that the coil is not energized and the aperture 101a is fully opened or it is closed, the rotor 104 is kept stopped by a holding force generated by magnetic holding means (not shown). In a state that the coil is not energized and a small-diameter aperture is defined by the diaphragm blade 120, the rotor 104 is kept stopped by the engaging portion 115a of the diaphragm actuating member 115 and the spring 116.
With the device disclosed in Japanese Laid-open Patent Publication No. 2001-117135, however, even when only the diaphragm blade 120 should be opened and closed, the shutter blades 118, 119 are opened and closed. This results in a problem that light incident through the aperture 101a is interrupted momentarily.